In a civil engineering construction site or a quarry site of a mine, various work vehicles such as a dump truck and an excavator are operated. In such work vehicles, the dump truck includes a vessel for loading a load thereon. Since the vessel stands to discharge the load, the dump truck increases in height when the vessel stands. For example, Patent Literature 1 discloses a technique of operating an alarm lamp and an alarm buzzer when a vessel's dumping state is detected upon operating a gear shifting lever for outputting a gear shifting signal to a running gear shifting device in order to avoid the vessel's contact and the like in a case where a dump truck enters a narrow space while the vessel stands.